Answer:
a. Codominant
Step-by-step explanation:
The R allele is codominant to the r allele.
In codominance, the two alleles that make up a trait/gene exert equal effects on each other and therefore contribute equally to the phenotypic expression of the trait.
r in homozygous state conditions a blood cholesterol level that is greater than 500 mg/dl.
R in homozygous state conditions a blood cholesterol level that is less than 200 mg/dl.
Heterozygous condition, Rr conditions a blood cholesterol level near 300 mg/dl.
Hence, r and R exerted equal effects on each other which neither being dominant nor recessive. A clear-cut case of codominance.
The correct option is a.